The present invention is related to a protection circuit for protecting a switching device that controls a current through an inductive load.
A protection device for protecting a switching device arranged to control a current through an inductive load subject to a supply voltage as well as a switching control device are already known in the art, e.g. from an STMicroelectronics product sheet as published on the Internet at http://us.st.com/stonline, which is herein incorporated by reference. Therein, a switching device, called a full bridge switching device with flyback protection consisting of power switches is shown. The switching device is protected with diodes.
The power switches of the switching device switch a supply voltage to a motor in such a way that an inductive load, e.g. a unipolar or bipolar stepper motor, starts running and increases the applied shaft rotation, by connecting a first terminal of this inductive load to a DC supply voltage and a second terminal to ground. The same principle scheme can be applied for DC motors, where the controlled torque can influence the motor speed directly.
It is further to be noticed that the shaft rotation torque applied to such a stepper motor increases as a function of the load current as long as a supply voltage is supplied. This goes on until resistive saturation occurs. To regulate the shaft rotation torque to a certain target level, the load current is regulated by connecting the inductive load (e.g. a motor) to a Direct Current supply voltage and then disconnecting it. This is done periodically, at a high frequency, in order to alternately increase the torque and subsequently decrease it in the same period. It keeps the average torque under control while in switched-mode operation.
This regulation is achieved for instance by reducing the current through the inductive load when an upper current level in the stepper motor is reached. This can be done by controlling the switching device in such way that the second terminal of the load is decoupled from ground, causing the voltage at the second terminal to increase (due to the inductive load) and to exceed the supply voltage, causing a flyback current through a (protection) diode.
The power switches used in the prior art are often discrete components, i.e. not integrated in a chip. In case an integrated circuit is used, intrinsic technological protection is provided or possibly external Schottky diodes are used.
If the switching device is integrated in a chip, the body diodes of the switches can be turned on during the xe2x80x98flyback phasexe2x80x99. Turning on these intrinsic body diodes creates substrate currents, which can lead to disturbances or xe2x80x9clatch-upxe2x80x9d. In case of latch-up, the circuit can be seen as equivalent to a thyristor structure that cannot be switched off and leads thereby to the destruction of the chip.
One method for avoiding high voltages at the output terminals is presented in U.S. Pat. No. 5,287,046. A method for controlling a chopper driver is disclosed in U.S. Pat. No. 6,119,046. The invention can be used in actuation electronics for stepped motors.
The present invention aims to provide a protection device for protecting a switching device arranged to control a current through an inductive load subject to a supply voltage.
One embodiment of the present invention provides a protection device for protecting a switching device arranged to control a current through an inductive load subject to a supply voltage, by switching a first and a second terminal of the inductive load to the respective pole of the supply voltage. The switching device is controlled by a switching control device, the switching control device operates based on a predetermined algorithm, and the switching device includes flyback body diodes. The protection device includes measuring means, decision means, and instructing means. The measuring means measures a voltage level on a first terminal of the load, and the decision means makes a decision on switching the first terminal of the load if the measured voltage level exceeds the supply voltage, to counteract the voltage level of the first terminal. The instructing means instructs the switching control device based on the decision on switching the first terminal of the load, to limit the current through a body diode by counteracting the voltage level over the load. In a preferred embodiment, the protection device comprises a comparator.
Another embodiment of the present invention provides a method for protecting a switching device arranged to control a current through an inductive load subject to a supply voltage, by switching a first and a second terminal of the inductive load to the respective pole of the supply voltage. The switching device is controlled by a switching control device, the switching control device operates based on a predetermined algorithm, and the switching device includes flyback body diodes. According to the method, a voltage level on a first terminal of the load is measured, and a decision is made on switching the first terminal of the load if the measured voltage level exceeds the supply voltage, to counteract the voltage level of the first terminal. The switching control device is instructed based on the decision on switching the first terminal of the load, to limit the current through a body diode by counteracting the voltage level over the load. Preferably, the method is implemented by use of a microprocessor of the switching control device.